atmel-mci: Fix memory leak in atmci_regs_show

The debugfs hook atmci_regs_show allocates a temporary buffer for
storing a register snapshot, but it doesn't free it before returning.
Plug this leak.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>